He said public transport users could become "oblivious to their surroundings" and miss signs of extremist behaviour.

"More than six million people travel on our railways every single day," he said.

"For commuters, who make the same journey over and over again, it can be easy to become oblivious to their surroundings.

"But I would urge them to remain alert, use their instinct and have the confidence to report anything that strikes them as out-of-place or suspicious."

Mr Crowther highlighted the case of Andreas Pierides, who was given an 18-month suspended sentence after admitting taking eight explosive flares on to a plane at a UK airport and possessing a terrorist manual.

The alarm was raised when a passenger, looking through a gap in a train seat, noticed Pierides was researching explosives and had instructions about how to make a bomb.

She instantly reported her concerns and police used CCTV to track him to the station where he bought his ticket. He was subsequently identified and arrested at Stansted Airport attempting to fly out of the country.